1. Field of the Invention
This invention relates to a technique for decoding an image coded in accordance with an image coding method adopting the discrete cosine transform (DCT) standard, and more particularly to a digital image decoding apparatus and method that is adapted to suppress or reduce the discontinuity in image blocks, referred to as the blocking effect.
2. Description of the Prior Art
Data compression is required for handling large amounts of information resulting from the recent multimedia tendency in communication media. Accordingly, there have been developed various information compression techniques. The most typical example in these information compression techniques includes an image coding method proposed by the motion picture expert group (MPEG). This MPEG coding method codes image data to a type of bit stream and supplies the coded bit stream to storage media or communication media, thereby reducing a transfer rate of data, a band width of the communication media, a storage space of the storage media and so on.
In the MPEG coding method providing the above advantages, the DCT is used as a basic principle of information compression. This results from the DCT having a tendency of concentrating frequency characteristics irregularly distributed on the field into the low frequency region. Accordingly, the MPEG coding method performs an operation called "quantization", in which the high frequency region is ignored after discrete cosine transforms, and thus is capable of reducing a loss of information to compress a picture efficiently. Further, in the MPEG coding method, the DCT is performed in a square block unit including a certain size of pixels, i.e., 8.times.8 pixels or 16.times.16 pixels, for one picture field. This DCT processing scheme acts as a factor that forces pixels in the boundaries of the square block to have discontinuous values in combination with the above-mentioned DCT characteristic of concentrating an information into the low frequency region. In other words, in the MPEG decoding method, there appears the discontinuity of image called "blocking effect" that makes a significant difference between values of pixels in the boundaries of a certain square block and those in the adjacent square blocks.
Image data compressed in accordance with the MPEG coding method adopting the DCT is decoded by means of a digital image decoding apparatus, as shown in FIG. 1, that includes a bit stream decoder 10, a memory 12 and a display 14. The bit stream decoder 10 performs the inverted quantization for a bit stream to derive high frequency components ignored upon coding and then performs the inverted discrete cosine transform (IDCT) of the inverted quantized data, thereby decoding the image data. The image data reconstructed by the bit stream decoder 10 pass through the memory 12 and is displayed on the display 14.
However, square blocks divided into a constant size as shown in FIG. 2 appear in the picture displayed by the digital image decoding apparatus as described above. This is caused by a fact that picture information concentrated in a mutually different frequency in a constant size of square block unit by the DCT upon MPEG coding is not reconstructed into its original form. This discontinuity of picture, that is, the blocking effect deteriorates a visuality of user and gives rise to "artifacts" in the boundaries of square blocks, thereby causing the user to feel an eye strain. For these reasons, the digital image decoding apparatus requires a function of suppressing or reducing the discontinuity of image or the blocking effects.